Semiconductor material such as silicon is used in either wafer or chip form in several types of optical devices. Some examples are the well-known infrared detector focal plane array formed on a silicon chip and another is a silicon liquid crystal light valve which uses a silicon wafer, and is of the type described in U. Efron et al, "A Silicon Photoconductor Based Liquid Crystal Light Valve", Society for Information Display Technical Digest, Vol. 12, 1981, page 142. The silicon chip used in the infrared detector focal plane array and the silicon wafer used in the liquid crystal light valve must have a flat finish of optical quality. To this end, particularly in the liquid crystal light valve, the silicon may be polished using mechanical-chemical techniques to achieve the optical quality flatness.
The substrate for the silicon based liquid crystal light valve described in the above-referenced Efron publication is a very thin silicon wafer, about 5 mils thick and about 2 inches in diameter, that is chemically and mechanically polished on both sides. Because the wafer is very thin and therefore somewhat flexible and because of the limitations of chemical-mechanical polishing, such silicon wafers do not have the ideal flatness preferred in liquid crystal light valves. Specifically, peak-to-valley deviations from flatness are typically on the order of 5 microns on each side. Because the flatness deviations on one side are independent of those on the other side of the wafer, the thickness variations may be as much as 10 microns.
For ideal optical performance of a liquid crystal light valve, the surface of the wafer should preferably be flat to within 1 micron.